Density Functional Theory Study of an Oxygen Reduction Reaction on

Mar 20, 2013 - Sung Joo Kim , Pei Lei , Kui Zhang , Chang Zhou , George W. Graham , and Xiaoqing Pan. Chemistry of Materials 2017 29 (5), 2016-2023...
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Density Functional Theory Study of an Oxygen Reduction Reaction on a Pt3Ti Alloy Electrocatalyst Shyam Kattel, Zhiyao Duan, and Guofeng Wang* Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States ABSTRACT: First-principles density functional theory calculations were performed to investigate the pathway of oxygen reduction reaction (ORR) on a Pt surface-segregated Pt3Ti(111) surface. Our calculation results indicate that the ORR intermediates (H, O, OH, O2, OOH, and H2O) would bind weakly on the Pt3Ti(111) surface compared to those on the pure Pt(111) surface. The possible ORR mechanism on the Pt3Ti(111) surface is elucidated by calculating the activation energies for all of the possible elementary reaction steps in ORR. We predict that the ORR on the Pt3Ti(111) surface proceeds via a H2O2 dissociation mechanism. Importantly, the activation energy for the rate determining step in the H2O2 dissociation mechanism on the Pt3Ti(111) surface is significantly lower than the corresponding value on the pure Pt(111) surface. This provides an explanation for the experimentally observed superior ORR performance of Pt3Ti electrocatalyst in comparison to pure Pt electrocatalyst. Furthermore, we studied the degradation behavior of Pt3Ti electrocatalyst by evaluating the electrochemical potential shift of surface Pt dissolution and the formation energy of a α-PtO2 layer on the Pt3Ti(111) surface. Our computations predict an enhanced stability of the Pt3Ti(111) surface against surface Pt dissolution and surface oxidation in comparison to that of the pure Pt(111) surface.

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INTRODUCTION Proton exchange membrane fuel cells (PEMFC) are actively pursued as a technology for power generation especially for nonstationary applications. However, the advancement of PEMFC demands the finding of high-performance, low-cost, and yet long-durability electrocatalysts. The high cost of precious Pt catalysts contributes significantly to the current price of PEMFC. Therefore, it is crucial to enhance the activity and simultaneously reduce the content of Pt during catalyst design in order to achieve large scale application of fuel cells. Currently, Pt-containing catalysts have the best combination of activity and durability for electrochemical reactions (for example, oxygen reduction reaction (ORR)) occurring in PEMFC.1−4 It has been found that alloying Pt with a 3d transition metal (TM) is an effective route to improve the activity of the catalysts for O2 electroreduction.1−6 This is because alloying Pt with TM provides a technical opportunity to tune the electronic properties of a catalyst for its optimum performance. In the meantime, the employment of Pt alloys, instead of pure Pt, reduces the use of Pt content in PEMFC. Consequently, interest inPt−TM alloys as ORR electrocatalysts has grown enormously in recent years. Particularly, Pt surfacesegregated bimetallic Pt3TM (TM = Ti, V, Fe, Co, and Ni) alloy catalysts have been shown to exhibit improved ORR activity as compared to pure Pt catalyst.3 For example, Pt-skin Pt3Ni(111) single crystal extended surface has been shown to possess 10 times higher ORR activity as compared to the pure Pt(111) surface.2 Moreover, Pt−Ti binary and Pt−Ti based ternary alloys show excellent ORR activity in previous experimental studies.7−9 Specifically, Pt3Ti nanoparticle electrocatalyst was found to have a 2-fold increase in the ORR activity as compared to the benchmark Pt/C elelectrocatalyst.7 © XXXX American Chemical Society

Furthermore, PtTiNi, PtTiCu, and PtTiV ternary alloy ORR electrocatalysts respectively displayed ORR activities with a 10fold, an 8-fold, and a 6-fold enhancement as compared to pure Pt electrocatalyst.9 The observed enhancement in the ORR activity was theoretically attributed to the fact that OH binds more weakly on the Pt-segregated Pt-TM surfaces than on the pure Pt surface.1,10−12 It is conceivable that alloying Pt with TM makes OH removal easier from the catalyst surface and hence increases the number of sites available for oxygen adsorption which is responsible for the observed increase in the ORR activity of the Pt-alloy elelctrocatalysts.2 A recent density functional theory (DFT) study suggested that the surfaces that adsorb O 0.0−0.4 eV more weakly than Pt(111) would show better ORR performance than pure Pt.1 To further elaborate the effects of TM addition in improving the catalytic activity for ORR, the detailed study about the energetics of ORR on the surface of thermodynamically equilibrated Pt-TM alloy electrocatalysts is quite useful. Besides activity for ORR, the long-term stability of cathode electrocatalyst in a harsh acidic environment is a serious challenge in the development of durable electrocatalysts for wide scale application of PEMFC. In PEMFC, the durability of metal catalysts is significantly affected by metal dissolution from the catalyst surface especially at high electrode potential13−15 and oxide growth at high O coverage.16 The dissolution of Pt from smaller particles and its redeposition on larger particles have been observed in experiments for Pt/C nanoparticle electrocatalyst after several potential cycling.17,18 The disReceived: January 5, 2013 Revised: March 11, 2013

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Ordered Pt3Ti bimetallic alloy adopts L12 crystal structure in which three Pt atoms lie at the face center positions and one Ti atom occupies the corner of a face centered cubic (fcc) unit cell. Our DFT calculated lattice parameter for the L12-Pt3Ti crystal structure is 3.956 Å which agrees well with a previous DFT value of 3.943 Å31 as well as with experimental value of 3.906 Å.32 The Pt3Ti(111) surface was modeled as a periodically repeated 2 × 2 super cell in three dimensions with four atoms per cell and four layers of atoms (Figure 1a)

solution process of metals in acid solution has been described using simple thermodynamic formalism based on DFT calculations by Greeley and Nørskov.19 In their computational approach, the thermodynamic electrode potential shift (ΔU) can be taken as a measure of the tendency to metal dissolution in alloy system in comparison to pure metal surfaces. Using this approach, Greeley et al. predicted a positive ΔU for Pt skin Pt3TM (TM = Fe, Co, and Ni) bimetallic electrocatalysts.19 However, if non-noble metals such as Fe, Co, and Ni are present on the catalyst surface, they will be dissolved easily into the aqueous environment because of their relatively low dissolution potentials. The issue of leaching transition metal out from Pt-TM alloys has been well recognized in the literature.20,21 In this regard, Ti has better corrosion resistance in different chemical environments.7 Previous experimental studies showed that PtTi bimetallic nanoparticle catalysts had better chemical stability in acid electrolyte than PtCo, PtNi, PtZn, and PtCu bimetallic electrocatalysts.8,22 The enhanced chemical stability of the PtTi catalyst has been attributed to the less reactive nature of Ti in comparison to Co, Ni, Zn, and Cu. Furthermore, a recent experiment revealed that PtTi bimetallic ORR electrocatalyst showed highest activity when the Pt−Ti nanoparticle had a Pt75Ti25 composition.7 PtTi-based ternary alloys such as PtTiNi, PtTiCu, and PtTiV exhibited multifold ORR activity enhancement as compared to the standard Pt/C electrocatalyst.9 Thus, PtTi-based alloys are promising candidates as durable ORR electrocatalysts in PEMFC. Consequently, we performed detailed first-principles DFT calculations to investigate the ORR mechanism on the Pt3Ti(111) surface and evaluate the stability of the Pt3Ti(111) surface. Previous studies have predicted that due to surface segregation a pure Pt outermost layer would form on the Pt3Ti(111) surface for Pt concentration slightly above 75 at. %, whereas the ordered Pt3Ti crystal structure would be maintained in the second layer and below.23 This prediction agrees with the earlier experimental measurements.24,25 Hence, in the present study, the thermodynamically equilibrated Pt3Ti(111) surface was modeled as a pure Pt outermost monolayer with an ordered L12-Pt3Ti crystal structure in the second layer and below. Specifically, we studied the adsorption of ORR intermediates (H, O, OH, O2, OOH, H2O, and H2O2) and further calculated the activation energies of all of the elementary reactions of ORR on this Pt surface segregated Pt3Ti(111) surface using a first-principles DFT method. Moreover, we investigated the corrosion properties of the clean and O adsorbed Pt3Ti(111) surface. Our theoretical results suggest that Pt surface-segregated Pt3Ti(111) surface not only has improved ORR activity but also possesses an enhanced durability as compared to pure Pt(111) surface.

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Figure 1. Atomic structure of the Pt surface-segregated Pt3Ti(111) surface model. (a) [110] projection and (b) [111] projection. In the figure, gray balls represent Pt atoms and brown balls represent Ti atoms. In panel b, various surface sites on the Pt surface-segregated Pt3Ti(111) surface are marked with letters.

similar to the previous studies.11 A 12 Å-thick vacuum was added along the direction perpendicular to the surface in the initial slab model to avoid the artificial interactions between the slab and its images. In all of the structure optimization calculations, the positions of the atoms in the bottom two surface layers were fixed while the positions of all the other atoms were fully relaxed. The thermodynamically equilibrated Pt3Ti(111) surface was modeled as a pure Pt outermost monolayer with ordered Pt3Ti crystal structure in the second layer and below (shown in Figure 1). The reported heats of reaction were calculated at 0 K. Both heats of reactions and activation energies include zeropoint energy (ZPE) correction. The ZPE correction is calculated as ZPE = ∑i (1/2)hνi where h is Planck’s constant and νi is the frequency of the ith vibrational mode of the adsorbate molecule. The vibrational modes were calculated explicitly under the frozen slab approximation. The vibrational frequencies were computed by displacing the adsorbate molecule by 0.005 Å in each of the three Cartesian directions and diagonalizing the resulting dynamical matrix.

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RESULTS AND DISCUSSION Surface Adsorption Sites. As discussed in the literature,10,11,33−37 there are top (T), bridge (B), fcc (F), and hcp (H) surface adsorption sites on the pure Pt(111) surface. Considering the Ti atoms in the second layer of our Pt3Ti(111) surface, there exist respectively two distinct kinds of fcc, hcp, and top sites and three distinct kinds of bridge sites on the outermost Pt monolayer. In Figure 1b, we marked out all of these different adsorption sites. F1 is an fcc site which has two Pt and one Ti atoms as nearest neighbors in the second layer, whereas F2 is an fcc site which has three Pt atoms as nearest neighbors in the second layer. Similarly, H1 is an hcp site which has a Ti atom underneath in the second layer, whereas H2 is an hcp site which has a Pt atom underneath in the second layer. Site T1 is the surface Pt atom which has two Pt and one Ti atoms as nearest neighbors in the second layer, whereas site T2 corresponds to the surface Pt atom which has three Pt atoms as

COMPUTATIONAL METHODS

In this work, spin polarized DFT calculations were performed using Vienna Ab-initio Simulation Package (VASP) code.26,27 All calculations were performed using ultra soft pseudopotentials,28 with a kinetic energy cutoff of 400 eV and with a FermiDirac smearing of 0.2 eV. Electronic exchange and correlation effects were described within the generalized gradient approximation (GGA) using PW91 functionals.29 The Brillouin zone was sampled on a regular 5 × 5 × 1 Monkhorst-Pack grid.30 Ionic positions were optimized until Hellman−Feynman force on each ion is less than 0.01 eV/Å−1. B

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Table 1. Calculated Binding Energies (in Units of eV) of Various ORR Chemical Species on the Pt3Ti(111) Surface Using DFT Methoda

a

chemical species

F1

F2

T1

T2

H O OH O2 OOH H2O H2O2

−2.65 −3.84

−2.28 −3.11

−2.46

−2.61

−2.04

−2.03

−1.05 −0.26

−1.01 −0.12

B1

B2

B3

−2.07 −0.39

−2.07 −0.41

−1.89 −0.39

−0.33

−0.42

−0.42

Letters F1, F2, T1, T2, B1, B2, and B3 refer to the surface sites marked in Figure 1b.

Figure 2. Atomic structure of the most stable adsorption geometries for various ORR chemical species adsorbed on the Pt3Ti(111) surface. (a) H adsorbed at fcc site F1, (b) O adsorbed at fcc site F1, (c) OH adsorbed at bridge site B1, (d) O2 adsorbed at bridge site B2, (e) OOH adsorbed at top site T1, (f) H2O adsorbed at top site T1, and (g) H2O2 adsorbed at bridge site B2. In the figure, gray balls represent Pt atoms, brown balls represent Ti atoms, red balls represent O atoms, and blue balls represent H atoms.

nearest neighboring Ti atom than at F2 site which has only Pt atoms as nearest neighbors. The binding energy of O2 has the lowest value (−0.41 eV) at the B2 site. Molecule OH could bind at both the T sites and B sites on the Pt3Ti(111) surface and form the most stable adsorption (with binding energy of −2.07 eV) at the B1 or B2 sites. Molecule OOH binds at the T sites with the O−O bond aligning along a bridge site, while molecule H2O binds at the T sites having the lowest binding energy of −0.26 eV at the T1 site and with H2O molecule parallel to the surface. The binding energies of H2O2 on the Pt3Ti(111) surface were the lowest (−0.42 eV) at the B2 and B3 sites. As compared with corresponding values on pure Pt (111) surface,11 our predicted binding energies for H, O, OH, O2, OOH, and H2O on the Pt3Ti(111) surface are slightly higher. Namely, these ORR intermediates would bind more weakly on the Pt3Ti(111) surface than on the pure Pt (111) surface. Our previous study revealed that the presence of transition metal Ti in the subsurface layers of the Pt surface-segregated Pt3Ti(111) surface would cause the d-band center of the outermost Pt surface layer downshift when compared with the one of pure Pt (111).23 As proposed in ref 39, such a change in electronic properties of surface Pt atoms is responsible for the weaker adsorption of the ORR intermediates on the Pt3Ti(111) surface compared to pure Pt(111) surface. Activation Energies of ORR Elementary Steps. The chemisorption of O2 on an electrocatalyst surface is the first step of ORR. The subsequent elementary ORR reactions can be summarized in terms of three possible ORR mechanisms (Figure 3).11 As shown in Figure 3, the ORR proceeds via an O2 dissociation mechanism if the chemisorbed O2 immediately undergoes O−O bond scission reaction to form 2(*O). Otherwise, the chemisorbed O2 participates a hydrogenation reaction to form *OOH which can undergo either O−O bond scission reaction to form *O and *OH in an OOH dissociation

nearest neighbors in the second layer. Moreover, site B1 is the bridge site between two surface Pt atoms at adjacent T1 sites and has a Ti atom as the nearest neighbor in the second layer, site B2 is the bridge site between two surface Pt atoms at adjacent T1 and T2 sites, and site B3 is the bridge site between two surface Pt atoms at adjacent T1 sites and has a Pt atom as the nearest neighbor in the second layer. Adsorption of ORR Intermediates. Using the DFT method, we determined the optimized configuration and the binding energy of each of the ORR intermediates (H, O, OH, O2, OOH, H2O, and H2O2) adsorbed on possible surface sites on the Pt3Ti(111) surface. According to previous studies,10,11,33,36 we focused on only studying the adsorption of H on the F and T sites, O on the F sites, O2 and H2O2 on the B sites, OOH and H2O on the T sites, and OH on both the T and B sites of the Pt surface−segregated Pt3Ti(111) surface. The binding energy of an adsorbate is calculated as the energy difference between the adsorbate−surface adsorption system and the systems of isolated clean surface and gas molecule. Thus, negative binding energy indicates attractive (energetically favorable) interaction between the slab and the adsorbates in this work. We list in Table 1 the calculated binding energies of the ORR intermediates at various adsorption sites, and plot in Figure 2 the optimized geometries of the ORR intermediates adsorbed on the Pt3Ti(111) surface. Our DFT calculation results indicated that H binds on the F and T sites. The most favorable binding of H at the F1 site has binding energy of −2.65 eV and that at the T2 site has binding energy of −2.61 eV. This suggests that H can occupy either T2 or F1 sites. Between the two possible F sites, the most preferable binding of O occurs at the F1 site with binding energy of −3.84 eV. It has been found that atomic species such as O would bind more strongly to Ti surface than Pt surface.38 In agreement with this result, we found that atomic H and O bound more strongly to the surface at F1 site which has a C

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Figure 3. Three possible ORR mechanisms: (a) O2 dissociation ORR mechanism, (b) OOH dissociation ORR mechanism, and (c) H2O2 dissociation ORR mechanism. In the figure, the asterisk (*) denotes the chemical species bound on the Pt3Ti(111) surface.

ORR mechanism or hydrogenation reaction to form *H2O2 in a H2O2 dissociation mechanism. Using the DFT method, we have calculated the heats of reaction (ΔE) and the activation energies (Ea) of all of the elementary reaction steps that constitute the above three ORR mechanisms and presented our results in Table 2. ΔE refers to

Figure 4. Atomic structures of the initial state (left panel), transition state (middle panel), and final state (right panel) for (a) O2 dissociation, (b) OOH dissociation, and (c) H2O2 dissociation reactions on the Pt3Ti(111) surface. In the figure, gray balls represent Pt atoms, brown balls represent Ti atoms, red balls represent O atoms, and blue balls represent H atoms.

Table 2. Calculated Heats of Reaction (ΔE) and Activation Energies (Ea) (in Units of eV) for Various Elementary Steps of ORR on the Pt3Ti(111) Surfacea Pt3Ti(111) reactions *O2 → *O + *O (Figure 4a) *OOH →*O +*OH (Figure 4b) *H2O2 → *OH + *OH (Figure 4c) *O2 + *H → *OOH (Figure 5a) *OH + *H → *H2O (Figure 5b) *O + *H → *OH (Figure 5c) *OOH + *H → *H2O2 (Figure 5d)

ΔE −0.92

Pt(111) ΔE

Ea 0.53

Ea

−1.02

0.63

−1.51

0.05/0.05b

−1.42

0.17/0.20

−1.79

0.47

−1.75

0.26

−0.25

0.20

−0.14

0.25

−0.76

0.12

−0.70

0.09

−0.37

0.80

−0.14

0.79

−0.43

0.07/0.13b

−0.28

0.19/0.22b

b

a

In comparison, the data on the corresponding ORR reaction energies on the Pt(111) surface (from ref 11) are also included. bActivation energies calculated using a 4-layer 4 × 4 super cell with 16 atoms per layer.

the enthalpy difference between the final state (products) and the initial state (reactants) of a chemical reaction while Ea is defined as the enthalpy difference between the transition state and the initial state of a chemical reaction. In this study, the initial state and the final state of chemical reactions were constructed based on our results of the adsorption of the involved chemical species while the transition state was located using the climbing image nudged elastic band (Cl-NEB) method40 in which the forces perpendicular to the tangent of the reaction pathway were relaxed to less than 0.05 eVÅ−1. The optimized geometries of the initial state, transition state and final state for O−O bond scission (O2 dissociation, OOH dissociation and H2O2 dissociation) and hydrogenation reactions (O hydrogenation, OH hydrogenation, O2 hydrogenation, and OOH hydrogenation) on the Pt3Ti(111) surface are shown in Figures 4 and 5, respectively.

Figure 5. Atomic structures of the initial state (left panel), transition state (middle panel), and final state (right panel) for (a) O2 hydrogenation, (b) OH hydrogenation, (c) O hydrogenation, and (d) OOH hydrogenation reactions on the Pt3Ti(111) surface. In the figure, gray balls represent Pt atoms, brown balls represent Ti atoms, red balls represent O atoms, and blue balls represent H atoms.

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energy for the rate determining reactions of ORR on the Pt3Ti(111) surface is significantly lower than the corresponding activation energy on the pure Pt(111) surface. This finding implies that kinetics of ORR should be more facile on the Pt3Ti electrocatalyst and explains the experimental observation that Pt3Ti electrocatalyst had better activity for ORR than pure Pt catalyst.3,7 The preferable ORR mechanism may vary with the coverage of the ORR species on the catalyst surface. To examine this point, we further performed DFT calculations for the reactions *OOH → *O + *OH and *OOH + *H → *H2O2 under low surface coverage using a 4 × 4 super cell (containing 16 atoms at each surface layer) of (111) surfaces. Our results in Table 2 indicated that, for these two reactions on both Pt(111) and Pt3Ti(111) surfaces, the activation energies calculated with a 4 × 4 super cell change only by a small amount as compared to the values attained with a 2 × 2 super cell. The largest change was found to be about 0.06 eV for the reaction *OOH + *H → *H2O2 on the Pt3Ti(111) surface. More importantly, it could be inferred that the ORR would still adopt the OOH dissociation mechanism on the pure Pt(111) but H2O2 dissociation mechanism on the Pt-segregated Pt3Ti(111) surfaces even with low surface coverage of *OOH or *H2O2. Dissolution of Pt from Pt3Ti(111) Surface. Following the approach in ref 19, the extent of the dissolution of Pt from Pt3Ti(111) surface relative to that from pure Pt(111) surface could be gauged with the electrode potential shift (ΔU) for reaction Pt → Pt2+ + 2e− on the two surfaces. In specific, ΔU = surf surf surf −((μsurf Pt3Ti − μPt,pure)/2e), where, μPt,pure and μPt3Ti are the chemical potentials of Pt atoms on the pure and alloy surfaces, respectively. Here, we assume that the dissolution of Pt atoms occurs only on the outermost layer of the catalyst surface. Thus, surf μsurf Pt,pure and μPt3Ti can be calculated as the energy difference between the surface slabs without and with a Pt atom removed from the outermost surface layer. It notes that due to subsurface Ti atoms the dissolution of different Pt atoms are not identical on the outermost surface of the Pt3Ti(111) surface. We found in the 2 × 2 Pt3Ti(111) surface model that removing the Pt surface atom which has three Pt atoms as nearest neighbors in the second layer (shown in Figure 6a) would cost lower energy than the other surface Pt atoms. Subsequently, we used this low energy bound to compute the electrochemical potential shifts between the Pt3Ti(111) and pure Pt(111) surfaces. Our DFT calculations predicted that ΔU for surface Pt dissolution from the Pt3Ti(111) surface was 0.11

For O2 dissociation (Figure 4a), the initial state is the adsorption of O2 at a B2 site and the final state is the coadsorption of two O atoms at the two adjacent F1 sites. In the transition state, one O atom stays at the bridge site while the other O atom moves toward an F1 site. For OOH dissociation (Figure 4b), the initial state is the adsorption of OOH at a T1 site and the final state is the coadsorption of OH at a T1 site and O at an F1 site. The transition state of this reaction corresponds to the movement of the O atom toward the F1 site and the OH to the nearby T1 site, accompanying the elongation of the O−O bond. For H2O2 dissociation reaction (Figure 4c), the initial state is the adsorption of H2O2 at a B2 site and the final state is the adsorption of the OH molecules at the two nearby top sites. In its transition state, the O−O bond in H2O2 elongates and the two OH groups rotate. The rotation of two OH groups causes O−O bond distortion in H2O2 which further facilitates the O− O bond scission. For O2 hydrogenation reaction (Figure 5a) the initial state is the coadsorption of O2 at a bridge (B1) site and H on a nearby top (T2) site and the final state is the adsorption of OOH at a top (T1) site. In its transition state, O2 molecule rotates toward H and H moves closer to O2. For OH hydrogenation reaction (Figure 5b), the initial state corresponds to the coadsorption of OH at a top (T1) site and H at a nearby top (T2) site and the final state is the adsorption of H2O at a top (T1) site. During transition state, OH molecule rotates toward H and H moves closer to OH. For O hydrogenation reaction (Figure 5c), the initial state is the coadsorption of O at an fcc (F1) site and H at a nearby top site and the final state is the adsorption of OH at a bridge (B1) site. In its transition state, O atom moves to bridge (B1) site and H is also displaced toward O. For OOH hydrogenation reaction (Figure 5d) the initial state is the coadsorption of OOH at a top (T1) site and H at a nearby top (T2) site. The final state is the adsorption of H2O2 at a bridge (B1) site. During its transition state H moves closer to O atom (in OOH) that forms a direct bond with surface Pt atom to form OH bond. It is noted that the attained transition states of ORR on the Pt3Ti(111) surface were similar to what has been previously observed on the pure Pt(111) surface.11 ORR Mechanism on the Pt3Ti(111) Surface. Our calculation results in Table 2 indicated that the activation energy for *O2 hydrogenation reaction was significantly lower than that for *O2 dissociation reaction on the Pt3Ti(111) surface. Hence, the O2 dissociation mechanism (shown in Figure 3) is predicted to be unfavorable for the ORR on the Pt3Ti(111) surface. Moreover, the activation energy for *OOH dissociation reaction was found to be higher than the activation energy for *OOH hydrogenation reaction on the Pt3Ti(111) surface. This result suggests that the ORR on the Pt3Ti(111) surface would proceed preferably via a H2O2 dissociation mechanism (shown in Figure 3). Furthermore, our study predicted that the rate determining step (i.e., chemical reaction with the largest activation energy along the proposed reaction mechanism) for the H2O2 dissociation ORR mechanism on the Pt3Ti(111) surface was *H2O2 dissociation reaction which had activation energy of 0.47 eV. In our previous work, we predicted that on the pure Pt(111) surface the rate determining step for the favorable OOH dissociation ORR was *O hydrogenation reaction with an activation energy of 0.79 eV.11 Consequently, we find that the calculated activation

Figure 6. Schematics of our atomic models for evaluating (a) Pt dissolution from a clean Pt3Ti(111) surface, (b) Pt dissolution from an oxygenated Pt3Ti(111) surface, and (c) α-PtO2 formation on the top of the Pt3Ti(111) surface. In the figure, gray balls represent Pt atoms, brown balls represent Ti atoms, and red balls represent O atoms. In panels a and b, arrows mark the location of the Pt atom removed from the surface. E

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layer and L12-ordered Pt3Ti crystal structure in below) as a cathode electrode electrocatalyst for promoting ORR in PEMFC. Our calculations predicted that the ORR intermediates (H, O, OH, O2, OOH, and H2O) would adsorb on the Pt3Ti(111) surface with weakened binding as compared to that on the pure Pt(111) surface. Through first-principles transition state calculations, we determined the activation energies for all possible elementary reaction steps of the ORR on the Pt3Ti(111) surface and further predicted that the ORR on the Pt3Ti(111) surface would proceed following the hydrogen peroxide dissociation mechanism. Moreover, we identified the rate determining step of the ORR on the Pt3Ti(111) surface to be the *H2O2 dissociation reaction with an activation energy of 0.47 eV, which is lower than 0.79 eV (previously calculated activation energy of the rate determining step for the ORR on the pure Pt(111) surface11). Hence, it was predicted in this study that the Pt3Ti catalyst should have higher activity for ORR than the Pt catalyst. This theoretical prediction agrees with the experimentally observed enhanced ORR activity of the Pt3Ti bimetallic electrocatalyst for ORR.3,7 Furthermore, we compared the stability of the Pt3Ti bimetallic catalyst with the pure Pt catalyst by computing the tendency of surface Pt dissolution and α-PtO2 oxidation surface layer formation. Our DFT results suggested that the Pt dissolution would take place at higher electrode potentials on both the clean and oxygenated Pt3Ti(111) surfaces than the corresponding pure Pt(111) surface. In addition, the formation of α-PtO2 oxidation layer was predicted to be less favorable on the Pt3Ti(111) surface than on the pure Pt(111) surface. Therefore, our study concluded that Pt surface-segregated Pt3Ti bimetallic electrocatalyst could have not only better activity for the ORR but also enhanced stability in PEMFC as compared to the pure Pt electrocatalyst.

V with reference to that from the pure Pt(111) surface. Positive ΔU indicates that the dissolution of Pt would take place at higher electrode potential on the Pt3Ti(111) surface as compared to the pure Pt(111) surface and, namely, that the Pt3Ti electrocatalyst would have better stability against Pt dissolution than the Pt electrocatalyst. Relevant to electrochemical environments of ORR, we also study the Pt dissolution from the oxygenated Pt3Ti(111) surface which contains an O atom adsorbed at the most favorable F1 surface site (Figure 6b). Generally speaking, Pt dissolution becomes easier from an oxygenated surface compared to a clean surface. For the Pt(111) surface, we predicted that the electrode potential shift (ΔU) would be −0.36 V between the oxygenated and the clean surface. Our prediction is in exact agreement with previous theoretical result of ΔU = −0.36 V for the Pt(111) surface.10 Similarly, our calculation showed an electrode potential shift ΔU = −0.34 V for the oxygenated Pt3Ti(111) surface relative to the clean Pt3Ti(111) surface. Comparing the oxygenated Pt3Ti(111) and Pt(111) surfaces, we found ΔU to be 0.13 V, which suggests that Pt3Ti electrocatalyst would still have better stability against surface Pt dissolution than the pure Pt electrocatalyst even under the oxidizing electrochemical conditions. Oxidation of Pt3Ti(111) Surface. Various transition metal oxide phases may be formed on the surface of Pt and Pt alloy electrocatalysts under oxygen-rich reaction conditions.16,41 Specifically, at high O coverage, the formation of PtO and PtO2 like structures has been reported on the Pt(111) surfaces.16,42,43 It is believed that the metal electrocatlaysts would lose catalytic activity if an oxide layer is formed and covered their surfaces.44 Moreover, it has been proposed that the Ti surface segregation in Pt−Ti alloys in the presence of adsorbed O could be possible as a result of the energetic competition between favorable Ti−O binding and unfavorable segregation of Ti to the surface.45 However, a recent experiment showed that oxidation of the Pt3Ti(111) surface to form a thin titanium oxide film would occur only above 500 K46 which is well above the operating temperature of a PEMFC. Hence, we only calculated and compared the formation energy of a α-PtO2 oxidation layer on the pure Pt and Pt3Ti(111) surfaces in this work. We modeled the α-PtO2 surface layer by adding one layer of O (four O atoms) at the fcc (F) adsorption sites above the outermost Pt layer and one more layer of O (four O atoms) at the hcp (H) adsorption sites beneath the outermost Pt layer of the Pt or Pt3Ti 2 × 2 (111) surface cell (shown in Figure 6c). After fully optimizing the structures, we calculated the formation energy of the oxidized Pt surfaces as the relative energy of our model (Figure 6c) with respect to isolated clean surfaces and gas oxygen molecules. In this way, the more negative our calculated formation energy is, the more susceptible to oxidation the catalyst surface would be. We predicted that the formation energy of the α-PtO2 surface layer was −1.82 eV on the pure Pt(111) surface and −0.87 eV on the Pt3Ti(111) surface. It could be inferred from our results that the formation of the oxide layer is more difficult on the Ptskin Pt3Ti(111) surface than on the pure Pt(111) surface, implying that Pt3Ti catalysts would be more stable against surface oxidation than pure Pt catalysts.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by Chemical Sciences Research Programs, Office of Basic Energy Sciences, U.S. Department of Energy (Grant No. DE-FG02-09ER16093). The computations were performed at the computer facility at the Center for Simulation and Modeling of the University of Pittsburgh.

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CONCLUSIONS First-principles DFT calculations have been performed to systematically evaluate the performance of the Pt surfacesegregated Pt3Ti(111) surface (which has a pure Pt outermost F

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